Production of iron sulphate



Oct. 26, 1943. H. E. KEYES PRODUCTION OF IRON SULPHATE 5 sheets-sheet 1Filed Nov. 17, 1958 l. i... t n n Filed Nov. 17, 1958 5 Sheets-Sheet 2 IN VEN TOR.

oct. 26, 1943. y H. E. KEYESY 2,332,647

PRODUCTION OF IRON SULPHATE Filed Nov. 17, 1938 s sheets-Sheet 3vP01/Ver Q se Power 4 FIG.

Q (D \g INVENToR.

Patented Oct. `26, 1943 UNITED STATES PATENT OFFICEl PRODUCTION oF moNSULPHATE Harmon E. Keyes, Phoenix', Ariz.. Application November 17,1938, Serial No. 241,023

8 Claims. (Cl. 231-126) In the elds of water and sewage treatment theneed for cheap iron salts is becoming of increasing importance. 'Ihemethod here described deals with the preparation and use of ironsulphate solution either inythe ferrous' or 'ferrie condition, the freehydrogen ion content, as determined bythe pH, being suitably controlledVby the same method.

Although the chloride form of iron has beenv extensively used inthe,past, yet it has been recently recognized by those skilled in theart that the desired result is accomplished by the action of the ferrousor ferric ion regardless of the anion with which the iron is combined.The criterion for determining the form of iron most suitable is thenthematter of cost. Because of this factand also that my method affords acheap and novel means oi manufacturing said iron salts at or, near thepoint of application this method is proposed for use in water treatmentor sewage disposal. However, I do not intend to limit this method ofproducingiron salts to the above fields and I lalso propose itsapplication to any other branch of industry having need for such salts.Specific reference to sewage is here made in order to demonstrate adefinite applicability.

The raw materials used are preferably elemental sulphur and anyconvenient form of scrap iron, such as tin cans reclaimed from garbageand which may be shredded or incineratecl before use. Sulphur dioxidemay also be produced by i any other means, such as roasting sulphide oreor use of sulphur bearing stack gases, or even liquid sulphur dioxide inspecial cases. as solutions, such as are formed in iron pickling Works,in copper eementation plants, or by dissolvingv metallic iron directlyby sulphuricv acid may also be used as a source of iron. A certainamount ofk air in conjunction with sulphur dioxide is vital to theprocess and is added'inany convenient'manner. Certain improved ways andlmeans of introducingv air and SO2 are included with this process as apart of this invention and which are applicable to other processesemploying iron sulphate or free acid. Another feature of this process iscontrol of pH of the resulting iron solution so that the desired resultis obtained when applying such solutions to their' intended use, Freeacid may be generated'as desired or it may be inhibited during orsubsequent to iron `oxidation so that an acidic or essentially neutralsolution of iron sulphate is produced at will.

Although a. plantof given dimensions has al maximum capacity foroxidizing iron or producing acid when the solution containsapproximately 10 grams iron per liter, yet I have shown that by mymethod of oxidation successful results are accomplished over a Widerange of iron` concentration, as forexample, vfrom less than 3 to morethan 50 grams per liter, the acid forming tendency being simultaneouslycontrolled or inhibited as desired. Iron oxidation is here carried outby the soealled aut-oxidation process, by which sulphur dioxide andferrous sulphate are simultaneously oxidized by air, and is shown by thereaction 2FeSO4+SO2+Oz=Fe2(SO4) 3 The acid forming reaction largelytakes place after iron oxidation has becomepractically complete,although some acid tends to form during iron oxidation. This acidformation is dependent on presence of ferrie iron, manganese or othermetal salts capable of acting catalytically, and was described byMelville Clark, British Patent No. 3,669, led March 9, 1888.

This contact with SO2 and air is conducted in a reaction apparatus,hereinafter described as the cell. Inasmuch Ias the conversion of SO2 ismore effective in hotthan in cold solutions I employ conservation ofheat inthe reaction apparatus and also utilize a certain amount ofsensible heat in the SO2 burner gas by suitable heat interchange or byadding SO2 gas at elevated temperatures directly to the solution. Coldiron solutions may thus be brought up to the desired temperature forthe. reaction with SO2 and oxygen. l

Certain means of effecting sulphur dioxide xation to produce' ferriesulphate are described in my previous patents, Nos. 1,823,831,1,952,675, and

2,055,082, and although such means could be employed with possibleadvantage in this process, yet asother means of conductingthis reactionare available and which'are not subject to the above` patents, theprocess here described may be conducted without regard to any specicmanner of iron oxidation or cell construction provided that the aboveaut-oxidation method is employed. However, novel and improved means ofconducting this iron oxidation, or acid forming, reaction are disclosedbelow as a, part of this invention.

One of the features of this process by which the previously knownaut-oxidation reaction may be used to produce a ferric iron solutionsuitable as a coagulent at the required pH is the inhibition of acid,thereby rendering it unnecessary to employ the large amounts of alkalinesubstance that would otherwise be necessary to neutralize the acidformed particularly when relatively strong ferrous sulphate solutionsare oxidized. By thus controlling the vacid forming tendency of thesulphur dioxide reaction I am able to pro-.- duce ferric sulphatesolution suitable for coagulating purposes. Acid inhibition during ironoxidation may be accomplished in a variety of ways.

The process may employ, but is not necessarily limited to certain meansof acid inhibition as are here described. The following methods ofrestricting acidity aregiven by way of example. (1) Manganese ore, of astate of oxidation higher than manganous and preferably manganesedioxide, may suitably be employed to both oxidize the remaining ferrousiron and neutralize acidity according to the reaction Thus, acid isconsumed and iron simultaneously oxidized. The manganese introduced intothe solution as sulphate may be beneficial by acting catalytically insubsequent oxidation steps or it may assist by actual fioc formation.This addition of manganese is conveniently carried out by addingmanganese dioxide ore in either lump or pulverized form to the SO2 cell,to the solution leaving the cell or to the portion of the solution beingadvanced to sewage treatment. If desired solution may bedrawn from thecell or storage tank and recirculated through a bed of manganese ore.Likewise, the advancing solution to sewage may be run through the bed ofmanganese dioxide. If the cell reaction is carried to the point ofinadvertently creating an excess of acid the reaction with manganese maybe balanced by adding the requisite amount of ferrous sulphate which maybe taken from the reduction circuit later to be described.' When anexcess of ferrous sulphate is present during the manganese oxidetreatment acid may be neutralized to the point of producing basic ferricsulphate without causing iron precipitation prior to adding to thesewage.

(2) Another means of inhibiting acid production by reaction with acompound of iron is use of ferrous carbonate according to the reactionBy contacting the'solution in the cell with iron carbonate the acid isinhibited by neutralizing as fast as formed. the ferrous sulphate thusproduced being oxidized in the cell. If -ferrous sulphate is desiredwith the ferric. as for odor conthe resulting iron hydroxide producedreturned to the cell .before it becomes inactive from loss of combinedwater. Actually, this iron hydrate compound may be complex in nature,but for sake of brevity its production may be shown by the reaction Ifdesired a separate charge of iron may be treated as above to produce ahydrate sludge which is thenadded to the cell circuit to inhibitacidity. Various ways of adding this sludge are shown in Fig. 5.However, if said sludge is not desired its presence may be practicallyavoided and loss of iron from insoluble compounds lpre-- vented byprecluding air from the reduction sys-- tem. This is incontradistinction to certain iron chloride processes in whichconsiderable loss of iron results from a hydrate-type of sludgeformation.

(5) Acid inhibition may be largely accomplished and without reactionwith any of the above type of chemical substances by my proposed memodof SO2 cell operation more fully described below. Briefly, thisvconsists'of sufficiently fine subdivision of Aair as to produce a densefrothing action with` the iron salts in the cell, by restricting therate of SO2 addition so that therefore follows that the same plant mayproduce both ferrous and ferric sulphate as final products in anydesired proportion. If only ferric sulphate, as for coagulation, isdesired and no appreciable acid is produced one third of the ferricsolution from the cell is advanced from the system and the remaining twothirds circulated over metallic iron, thus reducing the ferric andincreasing the dissolved iron content of the solution back to theoriginal total value according to the reactionFeSO44-(NH4)zS`=*FeS-1T(NH4)2SO4 t the entire ferric sulphate solutionfrom the cell is passed over metallic iron to give a neutral trol in thesewage plant together with coagulation, the ferrous carbonate treatmentcan be given to the advancing solution.

(3) In extreme cases the acid may be neutralizeddirectlybyj lime eitherin the cell or subsequently. but I generally prefer to utilize aneutralizing substance that reacts with acid to pro- -duce a useful formof iron such as above described.

.(4) Some of the metallic iron as is used in the dissolution circuit mayVbe converted to ferric hydroxide and added to the cell. Thus, thesolution is aerated during the reduction stage and ferrous sulphatesolution. In this case the quantity of iron advanced fromth'e'system tosewage equals onehalf-the amount oxidized at' 65 each cycle in the'cell.

From the above it follows that for every poundv of iron advanced in theferric condition three pounds are oxidized to ferric in the cell,wheresolutions containing at least 75 grams iron per l `tlo` which'.my lbe stated' asiV ,',Eniployinent vjofwgas at 10%. or

neness "bedy of froth persists on centrated ferrous sulphate solutionwhich can be transported from a central plant to points -of applicationa, reasonable distance away.

I have found that, whereas ferric sulphate may be reducedfto ferrousinless than 30 minutes by agitation with lnely divided metallic iron, suchas tin scrap, yet several hours may. be required for acidneutralization. If it is not convenient tosupply a sufficiently ne. formof iron or long enough contact time to complete the neutralization ofany acid present, I may employ neutralizing methods as above described`prior to treatment with metallic iron,thereby neutralizing the free acidin the entire' cell effluent. The greater reaction speed @of ferricsulphate as compared to free sulphuric acid in iron is desired, fo'rexample in transporting a considerable' distance, I find it expedient tooperate the cell so as .tot continue to build up a considerable acidconcentration, thereafter reacting'not only the ferric iron but also thefree 4' acid with metallic iron thus building up a higher concentrationof ferrousysulphate for sewagek or water treatment processes than wouldbe posl sible by simple reduction of the ferric iron.

In certain chemical processes for lsewage a pH less than '1, therequired 'acidity for the ferrie sulphate solution is readily producedin my processv by simple adjustment of the eell to a depth ofapproximately -30 per cent of the height of solution. Such froth, whichresembles that in a flotation cell, is indicative of the proper relationof the above factors Nand is a criterion for successful operatingconditions to produce solutions satisfactory for this process when suchfroth is formed under normal operat.

dissolving metallic iron is a feature of this procing conditions and iscaused to be produced -by the combination of factors as above stated.Furthermore, it may itself. perform a useful function in assistingoxidationand absorption of sulphur dioxide as the SO2 in being evolvedfrom the cell must pass through this froth column. A certain variationin degree of frothing is occasioned by change in state of ironoxidation, acidity, coalescence of bubbles or presence of frothingagents, which factors do -not vitally effect the cell operation althoughproducing a noticeable effect on frothing.JV The type of frothing that Ifind indicative of emcient cell operation is not materially 'inhibitedby any of the above conditions affecting frothing.

In the above connection I have found for example that satisfactoryoxidation is obtained by using standard porous aerators of No. 5 stand--ard porosity and which have a combined surface area approximately equalt that of the cell bottom. In a deep column uniformity of aeration maybe enhanced by placing porous aerators treatment in which occulation isobtained at i operation,` such as by extending the time of cenl tact gofSO2 beyond that required for ironv exi- L l typeof aeration cell may`,be employed as a step inrny process, yet I am'. hereproposing anddescribing a method of cell operation which I jhavefound to besuperiorto former schemes in that the rate of iron-oxidation and degreeof SOconversion are', greatlyY increased and the simultaneous acidfoririaitioxr'` decreasedf 'thus producing a solution highlysatisfactory asa rcol agulent and .,at-afreducedfcost as compared'toformer methods.l` f-,. o Y

-jThe criteria 'for successful cell operation -and proposed 'method 1will conformA may morey of SOzgas andl airfvented from'cell to A''contain '0.1 toom or ress- Vsoi; .1'0 tu 15 pounds iron oxidized? per 1poundoffacid simultaneously produced; 1rate of iron oxidation 6 to 12vgrmper liter per hour; strength iron solution 30-50 gm.` per liter; 98per cent oxidation offiron. I have discovered that the yaboveresults maybe simultaneousvly accomplished under such conditions of ofdissemination and rate of addition orl air-containing gases ,into thelower portions of the solution, uniformity of Aaeration, conditionsthroughout. the Jcell volume and concentration of,` iron in solutionthata substantial top, Q1 the solution and .companying drawings, Fig. l

at different depths in the solution so that fine bubbles are introducedin the'upper zones of the solution to counteract the coalescing tendencyof air introduced at the bottom. Uniformity of aeration is preferablysecured by spacing aerators at regular intervals in the cell. r e.

In conjunction with the above I have found that the sulphur dioxideadded in correct amount may be absorbed to give a conversion of 98-99percent Without use of any baffles, channels or specially deflectedsolution currents and at a pressure substantially less than thatemployed in the aerators, by simply introducing the S02 gas intothesolution slightly beneath the surface, using a perforated lead pipe oraeration diffusion tube. By'such means I can secure satisfactory celloperation with a solution column several feetv in depth and a pressureof SO2 gas only 10f-20 per cent that of the aeration air.

This permits use of moderate pressure blowers'of the .open blade typefor the sulphur dioxide gas and thus cheapens the cost of cell operationby not requiring positive pressure gas pumps or pressure typesulphurburners to force the gas under the same amount of solution headas is required for aeration.v However, if desired pressure type sulphurburners may be used, thereby eliminating SO2 gas compression andpermitting submergence of the SO2 absorber at any solultion depthcompatible with the air pressure used at the sulphur burner. l

-A distinctive feature of the method ofl sulphur dioxide fixation ashere vdescribed is that the gas discharged from 3the cell is notobjection'- abl'e as it may be S02 and a rate of on oxidation as high as10 gm. per liter per hour maintained. This is of importance in thatplants may be located Vclose to municipal districts where ldischarge ofsulphur containing stack gases in concentrations injurious to vegetationwould not be permitted,

'I'he steps of the processare shown in the ac being a ltypicalflowsheet'A for ferric sulphate land Fig. 2 for fer- 'rous sulphateproduction, A representing the SO2 conversion cell, B the system towhich the ld at less than 0.01 per cent iron salt is finally added and Cthe scrap iron reduction tanks. r

Although the drawings here given show the process operating on acontinuous basis, yet the batch system for either the oxidation orreduction phase may be employed .without deviating in any way from thespirit of this invention. For continuous operation of the oxidation cella steady flow of solution is maintained with a definite relation betweenthe quantity of iron oxidized and SO2 converted, using a cell preferablyof minimum size to aiord the required contact time. In batch'operationthe cell mechanism is preferably installed in a tank of relatively largedimensions so as to accommodate all the solution produced in a givenperiod of operation, as for one to 3 shifts, and the SO2 is shut oi whenthe desired amount of contact Iwith the solution has been obtained. Bothbatch and continuous type operation have' advantages peculiar vto agiven scale f operation or set of conditions. In general, for arelatively small scale operation or with fluctuating conditions of gasor solution ilow the batch system is preferable, while under steadyconditions and large scale operation the continuous method gives cheaperoperation. Obviously, for batch operation using a sufciently large cellto also function as a ferric solution storage tank the degree ofaeration in the tank for effective conversion of SO2 is much less thanin case of a tank of minimum required size, so that the aerator surfaceis also less in proportion to the size of the tank. This gives a lessdegress of frothing, butin general the sam'e principles as to 'iine airsubdivision apply though not to as large an extent, as in the case withthe continuously operating cell.

The above described method of cell operation is illustrated in Fig. 3, Ashowing a tank of any convenient size or shape; B porous aerators ofcylindrical or fiat design; C van absorber tube for dispersing SO2containing gas into the solution and which may 'be `a pipe perforated soas to handle a dusty gas as from sulphide ore roasting, or a porousdiifuser tube in case the gas is relatively clean; D is the solution andE the bed of froth indicating proper subdivision of air and rate ofaeration to eiTect the desired rate of conversion. By this type of cellSO2 conversion eftlciencies of 99 per cent or higher may be obtainedcoincident with an oxidation rate of approximately grams iron per literper hour and oxidation of iron to the point at which not over `0.1 gramferrous iron remains without forming an objectionablel amount of acid.

"Metallic iron or scrape in any suitable form may be used in thereduction stage. This I prefer to accomplish in a device shown in4- Fig.4 in which A is a tank, B an inlet pipe for ferric solution, C aperforated bottom holding.. the iron charge D, and E the discharge foradvancing efiluents. Recirculation of solution to provide additionalcontact with iron may be accomplished by any suitable pump or air lift,as Ashown by G. The solution may be thus recirculated, it may beagitated with air to' effect contact and produce ferric hydrate ifdesired, or the solution may be continuously advanced through a seriesof reducing tanks, thereby avoiding aeration conditions or necessity ofcirculating pumps.

One method of conducting my process is shown in Fig. 5. Here both ferricand ferrous sulphate are produced in'the same plant for distribution.This fiowsheet also illustrates the empolyment of ferric hydroxide (orhydrated oxide) to inhibit acid formation.

Inasmuch as the method here described con- I stitutes a complete processwith all necessary steps for production of ferric and/or ferroussulphate, any .mode of cell operation or construction may be adoptedWithout deviating from thi process in its essential features.

In actual operation, the products of my process are applied in anyconvenient manner. According to standard practice for sewage treatmentthe pH of the ferric solution is adjusted as above described to suit theindividual needs. If coagulation is desired by the action of hydrolysisthe acid in strong ferric sulphate solutions may be neutralized, as bymy method'of employing manganese dioxide, to the point of net basicityas shown by standard method of free acid determination, without causingprecipitationl of iron until dilution, thus lowering lime requirements.

The ferrous sulphate solution made as above described can be used incertain instances for water -,treatment when voxidation 'by' naturalmeans is feasible. The iioc is then formed following oxidation of theferrous iron by the dis,- solved oxygen in the water.

A further advantage of my method where use of appreciable acid isrequired is thatsuch acid may be generated at will in the cell and theobjectionable and dangerous handling of concentrated acid is entirelyeliminated.-

It is distinctly understood that in applying the ferric sulphate, asproduced by my process, to sewage treatment I am not coniined to anyparticular sewage treatment process or manner of addition of iron salts.Inasmuch as use of iron salts is generally recognized I propose the useof iron sulphate as produced by the method herel described, in any stageof the'sewage treatment to which such salts are applicable, for example,either in straight chemical precipitation or as an adjunct to activatedsludge.

It is further understood that the mode of cell operation as heredescribed may be extendedto produce free acid as desired in either acidsewage iloc formation or for industrial or metallurgical purposes. Forsuch use the iron concentration may be as low as 2 or 3 gms. per liter,or in special cases other catalysts, such as manganese, coulud beemployed. In such application I do not confine myself to any specialcatalyst or concentration thereof, but abide by the above describedcondition of cell operationby which the degree of aeration together withrequisite neness of air subdivision are evidenced by a copious andpersistent frothing, which condition increases the rate ofacid formationto a degree not heretofore obtained on a commercial scale.

Due to the sharp division point between the en d of the iron oxidationand beginning of the acid forming stage when my proposed method of celloperation is employed itis my purpose to proover 2 gm. per liter andwithout use of acid inhibiting chemicals.

In the adaptation of my process to situations in which automaticvcontrol is desirable, as for example inthe smaller municipal districtsnot having constant attendance at the sewage or water treatment plants,I propose as an adjunct to this process certain control features bywhich the SO2 and iron relations are so controlled that the degree ofiron oxidation and corresponding acidity in the solution are maintainedat a predetermined figure. As hasy been previously shown, it isimportant in ferric sulphate produc-- tion for coagulating purposes toclosely regulate pH conditions so that the resulting solution willfunction satisfactorily.l Although this may be accomplished by manualcontrol of the SO2 cell,

yet it is my purpose to make this automatic insofar as is feasible for agiven installation.

It is not within the' scope of this invention to prescribe tle definite'mechanical devices by which this control is accomplished, but rather todisclose the basic concept of utilizing 'certain functions in changes ofstate of oxidation or of pH such as conductivity or electrical potentialof the solution. By employment of available means of. measuring or,'indicating such functional changes in the solution, and with suitablerelay `and amplification devices I propose to electrically actuate vthemechanism controlling Ythe rate of sulphur feed, the iron solution owthrough the cell or the rate of addition of acid inhibiting agentswhichA last may be applied directly to the cell. or to the solutionfollowing the cell circuit.'v

A simpler installation may consist of signal or alarm devices by whichthe operator is warned of variation from a predetermined figure. In theabove connection'use of the "electric eye may ,c '5 yet due to thespacev occupied by the resulting froth the capacity'of` the cell iscorrespondingly reduced. I therefore propose to use in cases requiringmaximum output from a cell of" given size, certain means to minimizefrothing such as a circulating solutionspray; .Also, any chemical agenttending to inhibit frothing would be applicable for this purpose.

The requiste conditions for producing ferric sulphate for use ascoagulen't in alkaline or neutral circuit maybe summarized as follows: yl. Extremely ilne air subdivision for oxidation.

2. Adequate degree of aeration to produce copious frothing under normalconditions.

3. Avoidance ofl excessive dissolved SO2 in the cell solution by controlof SO2 addition rate so that air evolved from the cell generallycontains lessthan 0.1 per c ent SO2 by volume.

4..Addition of acid inhibitors if conditions 1-3 above do not producethe desired minimum of erating under suilicient pressure to contact thelgas with the solution, 4 the SO2 conversion cell,

5 a device either immersed in the cell solution or connected externallywith the cell solution and by which concentration .conditions in thesolution,

produce electrical changes which in turn actuate, as by the electriceyeor other sultabledevices, an electricrelay as shown by B'wliic'hfurlnishes current fromA thepower line to actuate the feed control mechanism1, 1a and lb, respec- K tively. Y-

Figure `6 represents a batch operatiomFig. 'lacontinuouslyoperating-'cell and Fig. 8 either a.

ling thev various feed devices the'apparatus mayvv acid. y

5. Stopping S05 reaction in cell at point of'` desired degree of ironoxidation and beforeobjectionable acid is formed.

By use of the automatic control features as shown in Figs. 6, 7 and 8,or even by manual control, my method of ferric sulphate production maybe applied to systems requiringv a definite and predeterminedpH, overthe entire range from neutrality to an acidity of 100 gm. free sulphuricacid per liter. 'I'his isl of importance in sewage treatment processesemploying occulation in acid circuit, as the acidity is generallyrequired to be maintained constantwithin close limits.

Examples A typical pilot plant installation and results obtained thereinare described by way of example. The SO2 cell held 154 gal. of solution,with a column height of 41/2 feet. The solution added to the cellcontained 32.85 gm. per liter ferrous and 33.8 total iron, the free acidbeing 0.29 gm. per liter. Air was added at the rate of 15.2 cu. ft. perminute free `air at a pressure of 3.05 lbs. per` sq. in., the Aairaddition being through two 'No.- 5 standard porosity .aerator tubesI setat the bottom of the tank. Satisfactory aeration is obtained by any.standard aerator giving air bubbles. approximately 1 millimeter, orless, in diameter. 'I'his is `conveniently'accomplished byl y thepreviously mntionedstandard alundum plate orv tube aerator` known asNo.5 porosity, specifthe'aerators andjwith'a submergenceo'f 1=6 inchesgot, solution. thus"y givingapressure onthe SOi j ilcations of which'lare given in catalog ofthe Carborundum Company. b'lhe Figure 5. denotes1a given finenes's of poreswhi'cfh y provides"the req- Y uis'itc'edegree of air dissemination with-a Vcorre-'q` spending lowresistanceconipatible with economical power consumption. Thejgasintroducedl to.`

l'the cell contained v1.6.7 per `'cent SOra'ndwa's laddedthrough:aii-ierforatedlead pipe set above bev simplifled by vusing theelectric circuit to 81V?" a signal or record the cell condition, theactual control of the variousfeed devices being .then conductedmanually. In each case .9 represents the porous aerators.

I have found that by extremely fine air subdi-- vision and adequatedegree of aeration. as evidencedby copious frothing as above described,that although the permissible rateqof iron oxidation without formingacid is greatly increased,

l fchemicai acid inhibitor being used. The solution temperature 'rosefrom 29 to 56 C. Ap-

proximately 50v gal." of solution was thus available -for advance tosewage treatment, the solution discharges from the cell contained 0.011per cent SO2 during theentirerur'i,y the conversion of SO2' being 99.7pendenti,l The ratio of oxygen in the aeration air to SOif-wasl duringthe run which balance beingV maintained by sending 100 gal. to thereduction system where by contact with shredded tin cans reduction was96.3 per cent complete in 1.42 hours, the reduced solution assaying 48.7gm. per liter ferrous iron, 50.65 total iron and 0.5 8qfree sulphuricacid. After 4 hours contact the reduction was 99.8 per cent complete.Air was blown into the reduction system and effected a certain amount offerric hydrate vprecipitation .which functioned in the next cycle toinhibit to some extent free acid formation.' For the next oxidationcycle water was added in such proportionas to strength and solutionvolume.

Similar runs were made in the same plant, but *employing manganesedioxide in the SO2 cell produce the original ironcircuit to inhibit acidformation. 'I'his was done j by circulating a small volume from the cellover a bed of manganese ore during the oxidationl y period.

Y free acid to the assay, the net free sulphuric acid in the solutionwas found to be minus 0.61 gm.

per liter which is quite within reason when it -is` considered thatcolloidal ferric hydroxide was kept in solution by the conditions thatobtained. This solution was stable, as no precipitation of iron occurred in a sample that stood for more than a month.

In the above examples the froth column stood t0 a depth of 15 inchesover the solution during of conditions without departing from the spiritof this invention, and that I am only bound by the limitations asimposed by the following claims.

I claim:

l. The process of: producing ferric sulphate consisting of contactingferrous sulphate solution with SO2 and air in a reaction chamber,diverting a portion of 'the ferric solution and contacting with metalliciron, adding air to the solution in contact with metalliciron to.thereby produce a suspension of hydrated iron oxide, adding saidhydrated form of iron together with ferrous sulphate solution producedby above metallic iron contact to .the SO2 air reaction chamber andreacting the hydrated form of iron to neutralize free acid formed by theSO2 contact.

2. A method of oxidizing SO2 to sulphate form which consists ofcontacting elemental sulphur and an excess of air under superatmosphericpressure said pressure being suihcient to overcome pressure resultingfrom the introduction of the gas lntoa solution body, burning said sull.phur to sulphur dioxide, coarsely dispersing the the entire oxidationperiod, the amount ofvsuch the last instance thus being 12.4 and 22.1times,

respectively, as great as in the first two cases that employed neaeration. Furthermore, in the last instance the SO2 conversion was only87.0 per cent, persistentfrothing being absent. To illustrate ability tooxidize very strong iron solutionsl a run was made under similar fineaeration conditions with a`solution containing initially 53.0 gm. perliter ferrous and 54.3 total iron. In 8.45 hours oxidation treatment theferric iron was 52.9 gm. per liter, the acid production being 3.5 gm.per liter. The ratio of oxygen to SO2 added was 6.04. 'I'he ratio ofpounds iron oxidized to pounds acid produced was 14.7, no acidinhibiting chemicals being used in this case.

It is understood that ferrous sulphate for use in my process could beobtained by'treating metallic ironwith commercial sulphuric acid, inwhich case recirculation of the cell solution to dissolve iron would notbe required. However, as formerly pointed out I prefer to employ theferric sulphate reduction step for iron dissolution, due to greaterreaction speed and freedom from the nuisance of handling concentratedacid.

It is furthermore understood that the various .features of this processmay be carried out in different manners and applied over a wide rangeresulting products of 4combustion beneath the surface of a body of aniron-containing solution and simultaneously and separately introducingfinely disseminated air into said body of solution.

3. A method of producing ferric sulphate comprising establishing a bodyof ferrous sulphate solution, separately and simultaneously introducinga mixture of gases containing SO2 and nely divided air into the solutionsaid mixture of gases containing SO2 being introduced directly into thezone of ne air dispersion in the solution; and

utilizing the air so introduced to agitate the entire body of solutionto produce a homogeneous solution` to assure a homogeneous condition ofthe solution body with respect to dissolved gases and metal salts,limiting the rate of introduction of SO2 and adding such excess of airto'maintain a preferential oxidizing action on iron sulphate, andneutralizing free acid in the solution prior to completion of ironoxidation.

4. In a process for converting sulphur dioxide to a sulphate radical inthe form of ferric sulphate, the steps that include introducing air inrelatively finely divided form into a body of an aqueous solution offerrous sulphate that acts in the presence of air to promote conversionof sulphur dioxide to a sulphate radical, in order to produce a mixturecontaining minute air bubbles dispersed in said solution; separately andsimultaneously introducing into said mixture in dispersed form a gascontaining sulphur dioxide to produce ferric sulphate in the solution;-and controlling the acidity of the solution during reaction by limitingintroduction of sulphur 'dioxide to a rate preventing formation of asubstantial concentration of free acid and stopping introduction ofsulphur dioxidewhen oxidation of `the iron is substantially complete.

5. In a process for making ferric sulphate, lthe steps that includetreating an acid ferrous sulphate solution to neutralize substantiallyall free dioxide when oxidation of the iron is substantially complete. d

6. In a cyclic process for converting sulphur dioxide to a sulphateradical in the form of ferricsulphate, the steps that includeintroducing air in relatively ilnely divided form into a body of anaqueous solution offerrous sulphate that acts in the presence of air topromote conversion of sulphur dioxide to a sulphate radical. in order toproduce a mixture containing minute air hubbles dispersed in saidsolution; separately and simultaneously introducing into'said mixture indispersed form a inixture ol' gases containing salts .of liron andmanganese, introducing al1-.in

sulphur dioxide to produce ferric sulphate in the solu'tion; `bringingsolution containing ferric suiphate into contact with metallic iron toreduce the ferric sulphate to ferrous sulphate and neutralize freeacida-returning the reduced solution to the original ferrous sulphatesolution; and

withdrawing solution from' the cycle and replacing it with make-upwater.

solution of lferrous sulphate that acts in the presy ence of air topromote conversion of sulphur dioxide to ferrie sulphate in order toproduce a solution homogeneous with respect todissolved constituents andcontaining minute air bubbles dispersed in said solution; separately andsimultaneously introducing into said solution a. mix- 7. A method of'oxidizing SO2 to form'sulphate 5 comprising establishing a body of asolution containing a dissolved metal salt of a type capable of actingcatalytically to induce the oxidation of dized.l S0: to sulphate fromthe group consisting of the Y l dition as soon ture of gases containingsulphur dioxide, and preventing formation of appreciable free acid bylimiting the rate of sulphur dioxide introduction to that amount whichwill combine to only oxi` dlze iron, and stopping said sulphur dioxideadas the iron lis substantially oxi- 'HARMON E.`KEYES.

Patent No. 2,332,647 Granted October 26, 1943 HARMON E. KEYES The aboveentitled patent extended October 2, 1951, under the provisions of theAct of June 30,` 1950, for 2 years and 168 days from the expiration ofthe original term thjereof.

11 Uonwm'ssz'omr of Patents.

